Microbeam’s Coal Quality Management System (CQMS) is a powerful software package for calculating advanced indices that are used to relate fuel properties to ash behavior in a coal-fired utility boiler and associated air pollution control devices. The image below illustrates indices and their specific application to boiler and air pollution control system performance based on fuel properties.
Fuel performance is estimated in terms of slag flow behavior, abrasion and erosion wear, wall slagging, high-temperature silicate-based convective pass fouling, low-temperature sulfate-based convective pass fouling, peak impact pressure for sootblowing, and ash cohesivity for fly ash handling as follows:
Slag Flow Behavior
T250: This value represents the calculated temperature at which the slag viscosity will reach 250 poise, based on ash composition. Given the fact that the upper limit of viscosity for slag fluidity is approximately 250 poise, the T250 value is generally used as the minimum slag removal temperature. (The cyclone temperature must be greater than the T250 in order for slag to flow from a cyclone boiler).
Strength Index: This index predicts the strength of deposited material. It is used in combination with the slagging index to assess slag deposit characteristics. Index values of less than 0.25 indicate weak deposits. Values of 0.25 to 0.34 denote low to moderate strength, and values of 0.34 to 0.41 indicate strong deposits. Index values greater than 0.41 correspond to flowing slag.
Cyclone Slagging Index: Provides information on the slag flow behavior in cyclones. This index is based upon partitioning of ash in the cyclone based on size and association of coal minerals. Standard partitioning criteria have been developed to provide the composition of the slag. Coal mineral composition is used to estimate the slag viscosity as a function of temperature. Index values range as follows: 1 (low viscosity), 1.5-2.5 (optimal viscosity), >3.0 (slag has the potential to freeze).
Wall Slagging Index: Indicates the propensity of deposits to accumulate on the radiant walls of a boiler, from 2000 to 3000°F. The slagging index is based on mineral size (especially illite, quartz, and pyrite), association of calcium (calcite can contribute to slagging), and viscosity of the silicate-based liquid phase. This index is used in combination with the strength index to assess slag deposit characteristics. Values range from 1-low to 20-severe.
Convective Pass Fouling
Silicate Index: Indicates the propensity of deposits to form from 1600 to 2400°F. This index is related to the formation of high-temperature fouling deposits in which silicates are the primary materials accumulating and the primary bonding component. Information used to derive the index includes the size of minerals such as quartz and clay, availability of alkali and alkaline earth elements, and the viscosity of the silicate liquid phase. This index is used in combination with the strength index. Values range from 1-low to 200-severe.
Sulfation Index: Indicates the propensity of low-temperature fouling deposits to form in the convective pass of the utility boiler from 1000 to 1750°F. This index is based on the availability of alkali (Na and K) and alkaline earth (Ca and Mg) elements to react with SO2 and SO3 to form sulfate, the primary material that causes particle-to-particle bonding in high-calcium coals. Sulfates are thermodynamically stable at temperatures below about 1650°F. Index values range from 1-low to 10-severe.
MTI utilized its database of over 8000 deposit and coal analysis to develop peak impact pressure index (PIP). The PIP index utilizes mineral type, size, and abundance along with proximate, ultimate, and ash composition to estimate porosity and glass-to-crystalline ratio of a deposit at selected temperatures. The tensile strength and PIP are related to the deposit porosity and glass-to-crystalline ratio. The potential for deposit removal using sootblowers can be related to the PIP index. The ability to remove deposits can be calculated from the properties of the fuels.
Abrasion Index: This index indicates the potential for wear of fuel preparation and handling equipment, as related to the hardness of minerals in the coal. The primary minerals of concern are quartz and pyrite. Values range from 0.1-low to 10-severe.
Erosion Index: This index indicates the potential for wear of boiler parts due to the impaction of fly ash particles, particularly those containing hard minerals such as quartz. The erosion index is dependent upon particle size and velocity. Values range from 0.1-low to 1.0-severe.
Inorganic Distribution Coefficient (Fine Particle Index)
The inorganic distribution coefficient (IDC) ranks coal based on the potential to form small particles. The IDC is based both on the ratio of total mineral to fine minerals, and on the quantity of organically associated elements. Lower IDC values indicate a higher potential to form more fine particles.
The resistivity of the ash has a significant impact on the collection efficiency of ash in electrostatic precipitators. High resistivity ash causes large voltage gradients across the ash layer on collector plates resulting in poor collection efficiency. The high resistivity ash significantly decreases the migration velocity of particles to the collection plate. Ashes with resistivity above 1010 ohm-cm cause poor performance. Bickelhaupt model is used to calculate resistivity as a function of temperature based on coal analysis and selected operational parameters. The resistivity varies depending upon the composition of the ash and the sulfur content of the coal.
Cohesivity provides an indication of the ability of fly ash to bond together at low temperatures. The index provides information on ash dust cake formation on ESP plate, the flow of ash in hoppers and on conveyers. The index is related to the composition of the fly ash as well as particle size.